The present invention concerns a display assembly allowing a chromatic contrast inversion to be obtained by means of two superposed display devices including a first display device in front of which is located a second display device formed by a particular arrangement of fixed polarisers and liquid crystal cells the switching configurations of which control the chromatic inversion.
The invention also concerns a timepiece provided with such a display arrangement wherein the first display device is formed by the dial of said timepiece.
The invention concerns more particularly a timepiece of this type wherein the dial includes an analogue display device of the current time on a dark background, the second display device being provided to supply other time-related or non time-related information, said information being able to be observed in two different colours.
A display assembly including two superposed display devices is already known from European Patent No. EP 0 926 574 in the name of the Applicant, the device oriented towards the observer being able to take two different states in which one of the display devices is visible to the exclusion of the other. Such a display assembly incorporated in a wristwatch is shown schematically in FIG. 1 and the operating principle thereof is given in FIG. 2.
The watch shown in FIG. 1, designated by the general reference 1, includes in a conventional manner, a middle part 2, a back cover 3 and a crystal 20 together delimiting a case in the bottom part of which are arranged an electronic watch movement 4 powered by a battery 5. Movement 4 includes an electronic time-keeping circuit associated with a drive device (not shown), hands 12, 14 and 16 for the hours, minutes and seconds, moving above a dial 18 carrying hour symbols (not shown).
Hands 12, 14 and 16 and dial 18 constitute the first display device, which is of the analogue type in the example illustrated, designated by the general reference 22.
This wristwatch 1 further includes a second display device of the digital type, designated by the general reference 24 and arranged between dial 18 and crystal 20. With reference again to FIG. 2, it can be seen that the second display device 24 is formed of a sandwich type structure including, moving from crystal 20 to dial 18, an absorbent linear polariser 40, a liquid crystal display cell 26, an absorbent linear polariser 42 crossed with respect to polariser 40, a liquid crystal optical valve 28 and a reflective polariser 44 crossed with respect to polariser 42. The switching states of the cell and the valve of the second display device are controlled by a control unit 23 as a function of manipulations effected on at least one external control member 9.
Display cell 26 includes in a conventional manner a transparent front substrate 30, a rear substrate 32 which is also transparent and a sealing frame 34 forming spacing and closing means delimiting with substrates 30 and 32 a closed cavity containing liquid crystals 27. The opposite faces of substrates 30 and 32 include transparent electrodes respectively 36 and 38 made for example of ITO. In the example illustrated, rear electrode 38 extends over the entire surface of substrate 32 and front electrode 36 is configured in digits which can be addressed separately by means of control unit 23, the activated (ON) or non-activated (OFF) state being symbolised by contactor 6 (FIG. 2). This cell 26 thus allowing alphanumerical characters to be displayed by passing liquid crystals 27 from a transparent state to an absorbent state, or conversely depending on the type of liquid crystals used. As will be understood hereinafter, in the application to a wristwatch where energy saving is of great importance, one will prefer to use liquid crystals which give the cell a transparent state when no voltage is applied and an absorbent state when voltage is applied.
Optical valve 28, containing liquid crystals 29 and having a contactor 8, has a comparable structure to that of cell 26 and differs therefrom only in that the two transparent electrodes totally cover the opposite faces of top substrate 31 and bottom substrate 33, so that valve 28 can pass from a totally transparent state to a totally absorbent state, or conversely depending upon the type of liquid crystals used. For the reason indicated previously, liquid crystals 29 will preferably be selected so as to have a transparent state in the absence of any voltage.
The examples disclosed in European Patent No. EP 0 926 574 only suggest, for the object sought by this invention, two switching configurations. In the OFF-OFF configuration, shown in FIG. 2, the reader will easily understand by following the path of the light rays through polarisers 40, 42, 44 and in an intermediate way cell 26 and valve 28, that the analogue display dial 18 is visible, whether it is light or dark in colour. According to a second configuration ON-ON, which is not shown, the display background is formed by reflective polariser 44, because of the rotation of 90xc2x0 imported by valve 28 on the polarised incident light, and the display digits appear in a dark colour because of the absorption by polariser 42 of the incident rays having undergone a rotation of 90xc2x0 passing through said activated digits of cell 26, so that the second display device appears in a dark colour on a mirrored background. This change of switching state from OFF-OFF to ON-ON allows an inversion of the display type to be obtained, but not a contrast inversion and even less so a chromatic contrast. The same would apply by modifying the relative orientation of the polarisers. In order to try to obtain a contrast inversion, even without any chromatic effect, those skilled in the art may naturally think of trying, without modifying the construction disclosed by the aforecited document, the ON-OFF configuration. With respect to the aforecited ON-ON configuration, the second display device is unchanged, and appears on the dial background instead of the mirrored background, but there is no contrast inversion.
The object of the present invention is thus to provide a particular arrangement allowing the colour of the displayed information or that of the background on which it is displayed to be reversed with another.
The invention therefore concerns a display assembly including a first display device in front of which is arranged a second active liquid crystal display device. This second display device is characterised in that it includes, moving from the exterior to the first display device a front first colour selective polariser, a liquid crystal dot-matrix or digit display cell, an intermediate second colour selective polariser, a liquid crystal optical valve and a rear polariser which is reflective if the first display device has a dark background, and absorbent if the first display device has a light background. A control unit allows two switching states to be selected for the cell and two switching states for the valve producing a chromatic inversion, either as regards the displayed information, or as regards the background on which said information is displayed. When the intermediate polariser is crossed with respect to the front polariser and with respect to the rear polariser, the chromatic inversion is observed as regards the information. When the intermediate polariser is parallel to the two other polarisers, the chromatic inversion is observed as regards the background. In order to obtain the best possible contrast, it is preferable to select two complementary colours for the front and intermediate selective polarisers.
According to another aspect of the invention, when the cell is not switched (OFF state) thus supplying no information, and as a function of the switching state (ON/OFF) of the valve, the display assembly allows the first display device only to be made visible or to be totally masked by a mirror mask. As will be seen in the following detailed description, this display assembly is particularly well suited to a wristwatch, since it is possible to have an OFF-OFF switching configuration which does not consume energy and which allows information as to the current time to be permanently visible on the first display device, and to be able to make two other types of information appear as required on the second display device, which can be differentiated easily by the chromatic inversion. This information can be complementary time-related information to those given by the first display device, or non time-related information such as values measured by sensor systems contained in the timepiece case.